As large-volume contents spread, demands have arisen for increasing the capacity of a magnetic recording apparatus such as a hard disk drive (HDD). Since the existing perpendicular magnetic recording method cannot unlimitedly increase the capacity, a recording method capable of further increasing the capacity is being searched for. With this background, research and development have taken place on a recording method called “microwave assisted magnetic recording,” which utilizes a magnetic resonance phenomenon in which a magnetic material resonates with a specific frequency. Also, a multilayer recording technique which increases the capacity of a magnetic recording apparatus by multilayering a recording medium has been proposed.
A magnetic recording apparatus uses a perpendicular magnetic recording medium having a large magnetic anisotropy constant Ku in order to prevent medium magnetization reversal due to thermal fluctuation. To reverse the medium magnetization of a perpendicular magnetic recording medium having a large magnetic anisotropy constant Ku, it is normally necessary to apply a high write magnetic field. The microwave assisted recording method involves resonating a magnetic recording medium by applying an oscillating magnetic field (also referred to as a microwave magnetic field) near the resonance frequency of the magnetic recording medium, thereby making a desired medium magnetization in the magnetic recording medium readily reversible, and performing magnetic recording. This makes it possible to reduce the magnitude of a write magnetic field.
In the research and development of the microwave assisted magnetic recording technique, a device using a spin torque oscillator is attracting attention as a microwave source for generating a microwave magnetic field. The spin torque oscillator is formed by a magnetic multilayer film similar to a GMR (Giant MagnetoResistance effect) element or TMR (Tunnel MagnetoResistance effect) element. More specifically, the spin torque oscillator has a basic structure including an oscillation layer in which magnetization can rotate, a pinned layer in which magnetization is fixed, and a spacer layer arranged between the oscillation layer and the pinned layer. In the spin torque oscillator, magnetization in the oscillation layer stationarily oscillates due to the spin transfer effect between the oscillation layer and the pinned layer. An oscillating magnetic field having a frequency of a few GHz to a few ten GHz deriving from this magnetization oscillation is generated near the spin torque oscillator. To extract an oscillating magnetic field having a large amplitude suitable for the microwave assisted recording technique from the spin torque oscillator, it is necessary to increase the thickness of the oscillation layer of the spin torque oscillator. However, the spin transfer effect is nonuniform in the thickness direction, and this makes it difficult to uniformly oscillate the oscillation layer having a large thickness.
The magnetic recording apparatus employing the microwave assisted magnetic recording is required to be able to apply a strong oscillating magnetic field in a write operation, in order to increase the microwave assisting effect.